It's almost impossible to crack down on foreigners working illegally in the sex industry because they have to be caught in the act, immigration authorities say.
While New Zealanders have been able to work as prostitutes since 2003, it is still illegal for immigrants to do sex work.
The Prostitutes Collective says because of that, they are being bullied, exploited and forced into work by unscrupulous brothel owners and clients.
Immigration New Zealand (INZ) said investigators had to find out where immigrants were working and then prove they had provided a sexual service.
Compliance and investigations area manager, Alistair Murray, said the legislation that decriminalised sex work for most prostitutes was accompanied by a tightening up of police enforcement powers on premises where they operated.
"So, as a result we end up with an environment where our ability to work there is contingent upon us knowing that a particular person is working there and shouldn't be," he said.
"Without a doubt it's definitely happening, but in order to establish that it's happening is quite involved.
"Not only do we have a limited ability to enter a property, what we are looking for is someone providing sexual services which is a difficult threshold to establish when you have someone who has paid for that - they're not going to be telling us that happened.
"Then you have the person doing the doing, who is our temporary migrant, they're not going to tell us that. So it is a difficult area to work in."
The Prostitutes Collective national co-ordinator Catherine Healy said the answer was to remove the exemption which stopped immigrants lawfully working as prostitutes.
She said it was brought in because of fears over sex trafficking, but the legislation's consequences were felt by the women it sought to protect.
They were being bullied and forced into work by brothel owners who knew the women were working illegally and felt unable to report their exploitation.
Foreign students who had turned to prostitution had lost thousands of dollars they had earned when they were reported to authorities and deported.
Other cases the collective dealt with were immigrant women leaving a violent relationship where their partner had a right to work but they did not, and even visitors who got into trouble.
"For example, two weeks ago two women contacted us, who had run out of money travelling around New Zealand and had been working to get their fare home," she said.
"They had their luggage and it was being withheld by the brothel operator, he wouldn't release the luggage and it had their passports.
"We had to actually call the police. Obviously witholding someone's passport is one of the indicators of someone who is not free to move and travel."
She said while the classified section of the newspaper might give the impression of an overwhelming number of overseas sex workers, that was unlikely to be the case.
"Some sex workers like to profile themselves as migrants, even though they have grown up in New Zealand, as it creates a point of difference for advertising."
'She was terrified'
Victoria University criminologist Lynzi Armstrong said she knew of a case of a prostitute being blackmailed by a client.
"She had had someone turn up at her door who did threaten to inform on her to Immigration and demanded that she provide him with free services.
"She was terrified and understandably so but she didn't want to report it and she didn't."
While some New Zealand prostitutes might feel resentful or threatened by overseas sex workers, many also felt a strong sense of solidarity, and believed visas should allow immigrants to work as prostitutes, Ms Armstrong said.
Immigration New Zealand said it worked closely with the police on finding illegal sex workers.
Officials stopped 312 suspected prostitutes from entering the country last year.
INZ said it did not keep figures on the number of brothels visited in a reportable format, nor did it know how many sex workers have been caught and deported.